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Tiny, highly venomous marine stingers are likely to have caused the deaths of two French tourists on the Great Barrier Reef near Cairns, a leading cardiologist has said.

Danielle Franck, 74, and Jacques Goron, 76, both died while snorkelling at Michaelmas Cay, about 33km north east of Cairns, on Wednesday.

Their deaths had been attributed to heart attacks, with the tour operator saying both had pre-existing heart conditions and swallowed seawater.

However, leading Sydney cardiologist Ross Walker told news.com.au dual fatal heart attacks occurring within just moments of each other, attributable only to pre-existing medical conditions were highly unlikely.

"When an older person gets bitten, it causes all sorts of issues," he said.

"To have a cardiac arrest at the same time, and the other a short time later, either they (the tourist operator) didn't think of the Irukandji jellyfish sting, which is bizarre because signs are everywhere and they are in the water from November until March, or they simply don't want to scare people."